As a conventional technique for performing contention control to avoid collisions of transmitted data using back-off control, there is CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) for use in IEEE802.11 which is a standard for a wireless local area network (LAN). In this conventional technique, when a terminal which performs data transmission and the like detects a state (hereinafter, referred to as an idle state) where a transmission line medium is not occupied by a transmission data frame (hereinafter, referred to as a transmission frame), the terminal does not perform data transmission immediately, and determines a transmission timing, in a contention zone defined in a contention window (hereinafter, referred to as CW), by back-off control using random numbers whose maximum value is the number of slots in the CW.
Specifically, when the terminal which performs data transmission detects a state (hereinafter, referred to as an occupation state) where a transmission frame occupies the transmission line medium by a transmission timing, the terminal waits to transmit a transmission frame until transmission of the transmission frame is completed. On the other hand, when the terminal which performs data transmission does not detect that the transmission line medium is in an occupation state by a transmission timing, the terminal transmits a transmission frame at this transmission timing. Then, when the terminal which performs data transmission does not detect a response frame (ACK frame) after transmitting the transmission frame, the terminal determines that a collision of the transmission frame has occurred, and performs a retransmission process. In this case, the terminal which performs data transmission reduces a probability of re-collision by increasing the number of slots in the CW to set the size of the CW to be large (a binary random back-off algorithm).
Here, generally, all terminals are required to equally obtain a chance of performing data transmission. However, in the above conventional technique, when collision of transmission frames occurs frequently due to the transmission line medium having high traffic (being congested), inequality that a terminal which newly demands to perform transmission can transmit a transmission frame with a higher probability than the other terminals occurs (known as a capture effect). This is because while the CW size of a terminal which repeatedly performs retransmission of a transmission frame is set to be large, the CW size of the terminal which newly demands to perform transmission is small. Further, with an increase in the CW size of each terminal, a waiting time of each terminal until transmission rapidly increases, resulting in a reduction in the throughput of each terminal.
The above problem is caused by each terminal setting its CW size in accordance with only the result of a trial of transmission. In other words, the above problem is caused by a fact that collision is not avoided with a state of occupation of the transmission line medium by a transmission frame (hereinafter, referred to as a medium occupation state) being taken into account in the whole system.
For the above problem, means for efficiently avoiding collision by an access point dynamically updating a CW size based on a medium occupation state and statistical information in each terminal in a system and the like and notifying each terminal of the updated CW size is disclosed (Patent Document 1 and Patent Document 2). In the Patent Document 1, an access point obtains a predicted value for the number of users (the number of terminals) in operation, and notifies each terminal of a CW size in accordance with the number of users in operation. Further, when collision of transmission frames occurs, the access point notifies each terminal of a CW size obtained by taking into account the number of collisions, thereby avoiding collision. In the Patent Document 2, because the traffic of an access point is larger than those of the other terminals, the access point monitors a quantity of packets remaining in the buffer thereof. When the quantity of packets remaining in the buffer increases, the access point facilitates handling of packets by increasing the CW size of each terminal.    [Patent Document 1] Japanese Patent Laid-open Publication No. 2002-374264    [Patent Document 2] Japanese Patent No. 3694844